Cholecalciferol (structure below), also called Calciol, is the mono-hydroxylated form of vitamin D, also called vitamin D3.

Cholecalciferol is produced naturally by solar UVB (290-320 nm) irradiation of 7-Dehydrocholesterol in the skin. After synthesis in the skin, Cholecalciferol is hydroxylated in the liver to 25-Hydroxycholecalciferol (Calcifediol) which is hydroxylated again in the kidneys to 1,25-Dihydroxycholecalciferol (Calcitriol), the active hormonal form of Vitamin D3. Cholecalciferol absorbs UV radiation with a peak at 265 nm and a molar extinction coefficient of 47,800. Its absorbance, though diminished in intensity after its peak, extends well into the solar UVB region (290-320 nm) as shown FIG. 2.
Cholecalciferol (Vitamin D3) is a member of the Vitamin D group of fat-soluble secosteroids, molecules in which one of the bonds in the steroid structure has been broken. In humans, the body can synthesize Vitamin D3 when sun exposure is adequate (hence its nickname, the “sunshine vitamin”). Unfortunately, skin protected from solar UV radiation by sunscreens is partly or completely prevented from producing Cholecalciferol. For that reason, it is often added to sunscreens and other skin care products in order to supplement its concentration in the epidermis where it is normally produced.
Following the final hydroxylation in the kidneys, calcitriol circulates as a hormone, regulating the concentration of calcium and phosphate in the bloodstream and promoting the healthy growth and remodeling of bone. Vitamin D prevents rickets in children and osteomalacia in adults, and, together with calcium, helps to protect older adults from osteoporosis. Vitamin D also affects neuromuscular function, inflammation, and influences the action of many genes that regulate the proliferation, differentiation and apoptosis of cells.
When exposed to sunlight, Cholecalciferol undergoes a destructive transformation of its double bonds to form a wide variety of photoisomers that have little if any biological activity on calcium metabolism. Therefore, the purpose of topically applying Cholecalciferol to skin—compensation for Cholecalciferol that is not produced in sunscreen-protected skin—is largely defeated, unless the Cholecalciferol itself is protected from photodegradation.
Vitamin D3—cholecalciferol—is the form of Vitamin D that is obtained through human exposure to direct sunlight. It is considered to be superior to other forms of Vitamin D such as ergocalciferol, which is better known as Vitamin D2. Research has shown that people in cold, rainy climates, are more susceptible to Vitamin D Deficiency, which has been scientifically linked to Seasonal Affective Disorder. This mean that people who live in places like Seattle, Wash., where the climate is often rainy and generally cold, are more likely to suffer from SAD, which is a type of seasonal depression that occurs primarily during the winter months.
A scientific link between low Vitamin D3 levels and depression has been established following several recent studies confirming the relationship. According to one such study by scientists at Georgia State University: The likelihood of having depression in persons with vitamin D deficiency is significantly higher compared to those with vitamin D sufficiency. Early diagnosis and intervention are paramount because coexistence of vitamin D deficiency and depression has serious negative consequences on health.
Vitamin D3 is produced in the skin when 7-dehydrocholestrol is irradiated by ultraviolet light (UBV) within the narrow wavelength limits of 290 to 315 nanometers. Once formed in the skin, it is carried to the liver where it is hydroxylated in a loosely regulated conversion to 25-hydroxy-cholecalciferol, then transported to the kidneys wherein, in a tightly regulated step, it is converted to 1,25-dihydroxycholecalciferol. Cholecalciferol has virtually no biological activity while 25-hydroxy-calciferol has roughly one one-thousandth the activity of 1,25-dihydroxycholecalciferol.
Although cholecalciferol clearly fits the definition of a hormone, which is defined as a substance produced in one body organ that exerts specific biological action in a target organ, it has long been classified as a vitamin because it is a small organic molecule that can be found in certain foods. It can best be described as a conditional nutrient, in the same category as coenzyme Q10, glutathione and glucosamine, which are small organic molecules that, under certain conditions, the body cannot synthesize in amounts sufficient to maintain health.
Compared to the current Daily Value recommendations of 200-400 IU per day for adults, 20 minutes of exposure to sunlight generates about 20,000 IU of cholecalciferol in a healthy adult. This is easily enough to avoid deficiency and build up the body's stores. Excessive exposure to sunlight does not lead to overproduction of vitamin D because continued sunlight exposure destroys vitamin D3.
Thus, the direct application to the skin of a composition containing vitamin D3, when the skin is exposed to sunlight, results in the vitamin D3 being destroyed by photoreactions, at the same time that additional vitamin D3 is being produced by the sunlight. Consequently, the packaging of the cholecalciferol product must be light fast. Even if cholecalciferol products are manufactured in the dark and stored in a light fast package, they quickly degrade upon application to the skin, rendering the cholecalciferol product much less effective.